Electrophotographic photoreceptors used in electrophotographic systems can be roughly grouped into inorganic photoreceptors mainly composed of inorganic photoconductive materials such as selenium and cadmium sulfide and organic photoreceptors mainly composed of various organic photoconductive compounds. Hithertofore, the inorganic photoreceptors, having superior sensitivity characteristics, have been used in high-speed copying machines. Their use, however, has been greatly restricted because of the toxicity of production materials and product compounds. In recent years, also from the viewpoint of environmental protection, such inorganic photoreceptors are strongly demanded to be replaced by harmless organic photoreceptors. In accordance with such an inclination, there is a strong demand for making the performance of organic photoreceptors higher. In particular, technical development for achieving a higher sensitivity is now an urgent subject.
Methods most commonly used for the improvement of performance of organic photoreceptors is the technique of function separation in which the function of carrier generation and the function of carrier transportation are separately assigned to different materials. Since the carrier generation and the carrier transportation are shared by different materials, it has become possible to select materials respectively suited for them from a vast range of materials. In particular, the organic photoreceptors, for which many kinds of compounds are available, are advantageous for achieving a higher performance by such function separation, and many carrier generation materials and carrier transportation materials are proposed.
The carrier generation materials for the organic photoreceptors include, for example, polycyclic quinone compounds as typified by dibromoanthanthrone, phthalocyanine compounds such as metal-free phthalocyanine and titanyl phthalocyanine, bisazo compounds or trisazo compounds, eutectic complexes of thiapyrylium compounds with polycarbonate, and squalium compounds, which have been put into practical use. The carrier transportation materials include, for example, pyrazoline compounds, polyalkane compounds, triphenylamine compounds, hydrazone compounds, tetraphenylbenzidine compounds, which also have been put into practical use.
In particular, the sensitivity characteristics directly depends on the performance of carrier generation materials, where the basic function thereof, i.e., the ability to absorb incident light to produce electron carriers not only depends on the molecular structure of the carrier generation material, but also is greatly influenced by the form in which the molecules thereof aggregate. For example, many crystal forms are known in the case of the above metal-free phthalocyanine or titanyl phthalocyanine. Since a difference in crystal form even with the same molecular structure makes the quantum efficiency of carrier generation entirely different, there is seen a great difference in the electrophotographic sensitivity brought as a result. As well known, X-type crystals or .tau.-type crystals of metal-free phthalocyanine show a sensitivity greater by one order than .alpha.-type crystals or .beta.-type crystals, and Y-type crystals of titanyl phthalocyanine shows a sensitivity greater by 3 to 4 times than A-type crystals or B-type crystals. Similarly, in the case of azo compounds and squalium compounds also, their sensitivities greatly change depending on differences in the structures of molecular aggregation.
Thus, in the further advancement of carrier generation materials, it is essential as factors to optimize their chemical structures and also to optimize the crystal structures or molecular aggregation structures.
A technique in which an imidazole perylene compound is used as a carrier generation material has been laid open in Japanese Patent Examined Publication No. 8423/1986 (U.S. Pat. No. 3,972,717). Since then, many electrophotographic photoreceptors making use of this compound as a carrier generation material have been studied. Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) No. 59686/1984 discloses a technique in which the compound is coated in the form of a dispersion to produce a photoreceptor, and Japanese Patent O.P.I. Publications No. 275848/1986 (U.S. Pat. No. 4,587,189), No. 180956/1988, No. 291061/1988, No. 186363/1992 and No. 186364/1992 disclose techniques in which the compound is used in combination with a specific carrier transportation material. Japanese Patent O.P.I. Publication No. 56444/1989 and No. 204850/1992 disclose techniques in which the compound is formed into fine particles by acid paste treatment, and Japanese Patent Examined Publication No. 41054/1988 discloses a technique in which a perylene type compound including this compound is purified by sublimation when used.
The fact that the imidazole perylene compound has some crystal forms is already reported in J. Imag. Sci., Vol. 33, pp.151-159 (1989), which discloses their X-ray diffraction spectra. However, what is mentioned in this publication with regard to electrophotographic performance is only concerned with deposited pigments and is unclear as to the relationship between the crystal forms and the electrophotographic performance.
Japanese Patent O.P.I. Publications No. 249719/1993 and No. 281769/1993 also disclose methods by which the crystal forms of the imidazole perylene compound are controlled. In such techniques, however, the crystalline state is controlled by dry process pulverization. Since a strong shear is locally applied during such dry process pulverization, there is a problem in respect of un-uniform pulverization to cause a disadvantage that black dots tend to occur in electrophotographic images. Also, in the dry process pulverization, a great mechanical impact is applied to the crystal powder, and hence crystal imperfections tend to be included, consequently tending to cause a lowering of the ability of charge potential retention. Thus, no satisfactory crystal control techniques are still available for the imidazole perylene compound, and the performance of the compound has not been brought out. In addition, all that have been disclosed in the above concerns the definition of crystal forms of a powder having not been dispersed (i.e., before dispersion), and by no means refer to the crystal forms of pigments having been dispersed (i.e., after dispersion) that have a direct influence on the electrophotographic performance. As will be made clear in Examples set out later, studies made by the present invention have revealed that, when photosensitive layer coating solutions are prepared using such imidazole perylene compounds, the X-ray diffraction spectra of the photosensitive layer coating solutions undergo changes depending on, besides the crystal forms before dispersion, the types of solvents, the conditions for preparation of coating solutions, e.g., dispersion strength, and the chemical purity of pigments, and at the same time cause changes also in electrophotographic performance. The relation with electrophotographic performance can not be said to be satisfactory unless the crystal forms of photosensitive layer coating solutions are studied.
The first object of the present invention is to produce a high-sensitivity organic photoreceptor, and has been achieved by employing as the carrier generation material the imidazole perylene compound having the specific crystal form.
Meanwhile, the carrier transportation material used in combination with such a carrier generation material is also an important factor on which the performance of photoreceptors depends. Not to speak of sensitivity performance, the properties of photosensitive layers that are required when used in photoreceptors greatly change depending on the carrier transportation material. Japanese Patent O.P.I. Publications No. 249719/1993 and No. 281769/1993 disclose techniques in which a benzidine type carrier transportation material is used. However, photoreceptors produced by incorporating such a compound tend to cause microscopic fissures (herein called cracks) in photosensitive layers, and have the problem that faulty images due to cracks tend to appear.
The second object of the present invention is to produce a photoreceptor that can be free of cracks, and has been achieved by the selection of the specific carrier transportation material
As a constituent of the photoreceptor of the present invention, the photoreceptor has the intermediate layer in addition to the carrier generation layer and the carrier transportation layer. The intermediate layer is positioned between the conductive support and the photosensitive layer (comprised of the carrier generation layer and the carrier transportation layer), and is formed to have the function, e.g., to bond the support and the photosensitive layer, to cover defects on the support, to prevent insulation failure of the photosensitive layer from being caused by a charging assembly and to prevent unwanted charges from being injected from the support, and is provided, for example, by forming on the support a layer comprised of a polymeric compound such as polyvinyl alcohol, ethyl cellulose, carboxymethyl cellulose, an ethylene/vinyl acetate copolymer or casein. At any event, there has been the problem that the surface potential of photoreceptors may change because of a high residual-potential and a change in electrical resistivity due to environmental variations.
The third object of the present invention is to produce a photoreceptor that can be free of faulty images such as fog and blank areas and also may cause less changes in electrophotographic performance against environmental variations, and such an object has been achieved by the selection of the specific materials for the intermediate layer.
One of essential factors of photoreceptors is the conductive support. There are defects such as scratches and irregularities on the surface of supports having been just injection molded. If the photosensitive layer is formed on such a surface, faulty images such as pin-holes and black dots tend to occur at the time of image formation. Accordingly, supports whose surfaces have been mirror finished by means of a diamond tool or the like have been widely used in conventional analog copying machines. However, in recent years, with a progress of electronic equipment, there is an increasing demand for printers and digital copying machines making use of semiconductor lasers as light sources. When photoreceptors employing the supports having been subjected to such mirror finishing are used in printers or copying machines, conspicuous interference fringes (moire) tend to appear at halftone image portions.
The fourth object of the present invention is to produce a photoreceptor that may cause very less interference fringes even when laser beams are used, may cause no faulty images such as blank areas, black dots and density decrease and may cause less lowering of image quality and less deterioration of sensitivity even in its long-term use. Such an object of the present invention has been achieved by employing as the carrier generation material the imidazole perylene compound having the specific crystal form.
Electrophotographic photoreceptors have conventionally the constitution that a metal support made of, for example, aluminum, copper, brass, steel or stainless steel, or a plastic support on which a metal thin film of aluminum, palladium, gold or the like formed by lamination or deposition to impart conductivity, is provided thereon with a low-resistivity thin-film intermediate layer, and a photosensitive layer is provided on the intermediate layer.
The intermediate layer is formed to have the function, e.g., to bond the support and the photosensitive layer, to cover defects on the support, to prevent insulation failure of the photosensitive layer from being caused by a charging assembly and to prevent unwanted charges from being injected from the support, and is provided, for example, by forming on the support a layer comprised of a polymeric compound such as polyvinyl alcohol, ethyl cellulose, carboxymethyl cellulose, an ethylene/vinyl acetate copolymer or casein, but is still unsatisfactory.
If the intermediate layer is formed in a large thickness, carriers are hindered from being moved to the support side at the time of exposure to light to cause an increase in fog and a deterioration of image quality, and hence the intermediate layer is usually formed in a thin layer of 0.1 to 2 .mu.m thick.
The formation of such a thin layer imposes a difficulty in operation on account of manufacture techniques, and it is difficult to carry out uniform operation, where support defects such as scratches, irregularities and dust on the support surface can not necessarily well covered to tend to faulty images such as streaks, pin-holes and black dots at the time of image formation.
Accordingly, it is common to use supports whose surfaces have been mirror finished by means of a diamond tool or the like.
When, however, the supports having been subjected to such surface treatment are used in copying machines, printers or the like making use of semiconductor lasers as light sources, the state of the support surface may affect images to tend to cause conspicuous interference fringes (moire) at halftone image portions to greatly damage the images
As a countermeasure for the moire, it is well known to use a technique in which the surface roughness of the support surface is defined to prevent interference fringes from occurring.
If, however, the support whose surface is in a rough state is used as in the above countermeasure, the support surface has a poor cleanability compared with the mirror-finished support when the support is cleaned. Hence, aluminum cuttings, dust, dirt, cutting oil and so forth having not been completely removed by cleaning may remain on the surface. These may cause blank areas, black dots and density decrease, resulting in faulty images, when images are formed by development in copying machines and printers making use of semiconductor laser light sources.